A centrifuge of substantial size, typically one handling hundreds of gallons or more per minute, normally requires two drive mechanisms. One drive is connected to the bowl and rotates the bowl at a specified speed. Another drive is connected to the internal conveyor which is an elongate screw with one or two flites. The screw is run at a speed which is close to the bowl speed, but slightly offset from it. Depending on the lead or lag of the screw flites, and depending on the nature of the separation being accomplished in the centrifuge, the screw is operated at a few rpm difference in speed.
The power required for operation of the centrifuge is in part defined by the volume of material being rotated. Effectively, this requires calculation based on the diameter and length of the bowl which, for this calculation, is deemed to be filled to maximum capacity with the material undergoing separation. Most of these mixtures are suspensions which require separation of sediment or perhaps emulsions of oil and grease which are separated into separate water and oil phases. Whatever the circumstance, the energy required for operation is a significant factor. Moreover, for a given mode of separation, the operation of the centrifuge has to be adjusted. Sometimes, this involves increasing the rpm. When the rpm increases, the slurry undergoing separation at the outer wall is subjected to maximum gravitational pull, thereby speeding up the separation. For instance, with an emulsion of water droplets in oil or oil droplets in water, the difference in specific gravity can be quite small. Such emulsions exist because the specific gravity difference is so small that surface tension overcomes it to prevent gravity separation to the bottom. Without the assistance of a centrifuge, merely placing an emulsion in a container and waiting a long time will ordinarily not produce much separation, if any, because time only cannot overcome the surface tension that keeps the drops or bubbles floating. Accordingly, the use of a centrifuge accentuates the modest difference in specific gravity or density. If the density difference is negligible, when exposed to a centrifuge imparting a pull of 2,000 or even 3,000 g's, separation can be achieved. Obtaining that level of pull, however, requires greater performance from a centrifuge.
Centrifuge operation is not merely the concept that more power accomplishes a better separation. If, in a given situation, the centrifuge is operated at speeds imparting 3,000 g's to the slurry, then increasing that to 4,000 g's will not necessarily provide a better separation. A better separation is not necessarily obtained by increasing the rate at which the screw conveyor removes the separated particles in the bowl. There are interlocking difficulties with regard to all of these operations. More than that, there are capital costs of equipment for the implementation of such controls to the end that better separation is normally obtained with a dual motor setup. Elaborating, a large motor is necessary to rotate the bowl. Assume for purposes of description that the large motor is a 100-horsepower motor. A smaller motor, perhaps 40-horsepower, is required to operate the conveyor. An elaborate control system between the two motors is necessary to operate them at the selected speed differential. That setup might provide the optimum separation, but it describes the more costly of structures and is remarkably complex in the sense that two separate motors must be controlled and the operation of the two must be correlated. For instance, when the conveyor is off, switching on the bowl motor must be timed with respect to switching on the conveyor motor, and they must come up at related speeds as they approach the normal operating speeds required for them. Otherwise, the beginning moment will involve a defective separation, i.e. a separation which does not accomplish the intended purpose, and all the product through the machine at that startup circumstance will not meet the desired specifications.
It is not uncommon for a large centrifuge of the type just mentioned to cost $200,000 to $400,000 in 1998 prices. The power consumption of the two separate motors is quite large. When installed, and operated around the clock, the two motors just mentioned will consume electrical power of great value. The power consumption aspect is just as significant. Accordingly, the present disclosure sets forth an improved centrifuge system which is capable of providing the desired or optimum separation. This is done with a simplified system reducing the number of electric motors from two to one, and this is done with a system which is able to accomplish optimum operation. Optimum operation is denoted by adjustment to the proper speeds. The system of this disclosure has advantages which derive from simplification. A single motor starter and associated electrical power circuit is needed, not two. Moreover, the system enables adjustment to the desired speed difference to prompt the conveyor to rotate at the correct or desired speed.